Many proposals have been made on an ion exchange membrane type alkali chloride electrolytic cell for producing highly pure, alkaline metal hydroxides with a high current efficiency and a low voltage. Among them there are proposals concerning a zero-gap type in which an anode and a cathode are in contact with each other with an ion exchange membrane interposed therebetween.
U.S. Pat. No. 4,444,632, JP-B-6-70276 (corresponding to U.S. Pat. No. 4,615,775 and European Patent No. 124125) and JP-A-57-98682 (corresponding to JP-B-1-25836, U.S. Pat. No. 4,381,979 and European Patent No. 50373) have proposed electrolytic cells using wire mats. Japanese Patent No. 2876427 (corresponding to U.S. Pat. No. 5,599,430) has proposed a mattress for an electrochemical bath.
Some of these patents have an expanded pressure plate and a cathode fine mesh screen. In these electrolytic cells, however, the mat strength, anode shape, electrolyte concentration distribution or in-cell pressure variations are not appropriate, which in turn gives rise to problems of an undesirable increase in voltage and breakage of the ion exchange membrane.
JP-B-5-34434, JP-A-2000-178781, JP-A-2000-178782, JP-A-2001-64792, JP-A-2001-152380 and JP-A-2001-262387 disclose elastic mats and the strength thereof. These references also disclose the strength of cathodes and a way to prevent collapse of the mats.
These improvements are effective. However, at a high current density of more than 5 kA/m2, the improvements are not enough for electrolysis with a stable long-term current efficiency and voltage.
Other zero-gap electrolytic cells use springs. For example, JP-A-10-53887 discloses an electrolyzer using a spring. However, the spring increases pressure in local areas and may cause damages to a membrane in contact with it. Electrolyzers that can employ the zero-gap structure are shown in, for instance, JP-A-51-43377, JP-A-62-96688 and JP-A-61-500669 (corresponding to WO85/2419).
These unit electrolytic cells have no air-liquid separation chamber formed within them and extract gas and liquid upwardly as is in an air-liquid mixed phase. This causes vibrations in the unit electrolytic cells and gives rise to a problem of possible breakage of the ion exchange membrane. Further, they have no provisions inside for mixing electrolyte and have a problem that a large volume of electrolyte has to be circulated to evenly distribute the electrolyte within the electrolytic chamber.
JP-A-61-19789 and JP-A-63-11686 disclose a way to extract gas and electrolyte downwardly rather than upwardly. However, gas and liquid may in some cases be drawn out in a mixed phase, making it impossible to prevent vibrations inside unit electrolytic cells. Further, a conductive dispersion member or current distribution member intended for internal circulation of the electrolyte is provided to make electrolyte concentration uniform in the cells, but this has a drawback of making the electrolyte cell structure complex.
JP-U-59-153376 discloses a wave elimination plate as a countermeasure for preventing vibrations in an electrolytic cell. This alone, however, can not provide enough wave elimination effect, and it is impossible to completely eliminate vibrations caused by pressure variations in the electrolytic cell.
JP-A-4-289184 and JP-A-8-100286 disclose a cylindrical duct and a downcomer for internally circulating an electrolyte to make the electrolyte concentration uniform in the cells. This, however, makes the structure in the electrolytic cells complex and increases the manufacturing costs. Further, for electrolysis at a high current density of more than 5 kA/m2, the electrolyte concentration distribution is still large enough to have possible adverse effects on the ion exchange membrane.
Furthermore, although these publications attempt to prevent vibrations by (1) providing an air-liquid separation chamber having a relatively large volume and by (2) extracting gas and liquid in a separated state downwardly or horizontally, vibrations may still occur in some cases at a high current density of more than 5 kA/m2.